Chemistry of posphorus adsorption in major soil types of Kerala

Abstract

Phosphorus is a vital nutrient crucial for plant growth and development. However, it is often unavailable for plants due to different reactions of applied phosphorus in soil. The favourability of these reactions depends upon the physico-chemical and pedogenic properties of soil. The tropical climatic conditions of Kerala and the dominance of kaolinitic clay minerals coupled with the leaching of basic elements and accumulation of sesquioxide, lead to the high adsorptive capacity of the soil. In managing P availability in the soils of Kerala, with consideration of environmental aspects, a basic study regarding P adsorption is necessary. Hence, the study entitled “Chemistry of phosphorus adsorption in major soil types of Kerala" aims to elucidate the chemistry behind phosphorus adsorption in soil. Georeferenced surface soil samples were collected from 27 locations in 9 agro-ecological units of Kerala (North central laterite (AEU-10), Palakkad eastern plain (AEU-23), Kole lands (AEU-6), Northern coastal plain (AEU-2), Pokkali lands (AEU-5), Kuttanad (AEU-4), Onattukara sandy plain (AEU-3), Wayanad central plateau (AEU-20) and Kaipad (AEU-7)) and characterized for physicochemical properties The majority of soils collected from lowland soils were deficient in P, while soils from AEU-2 and AEU-3 were high in P availability. Phosphorus fixing capacity had a significant positive correlation with OC, silt, clay and FeCBD and a negative correlation with pH, POX and available P. An adsorption and desorption experiment was conducted to investigate the behaviour of soil concerning added phosphorus. The L-shaped Q-I curves of Pokkali, Kole, and Kuttanad indicate the high affinity of the adsorbent for the phosphate ions at low concentrations, which then decreases as concentration increases. Langmuir, Freundlich and Temkin isotherms could explain phosphorus adsorption in soil. The best fit of collected soil samples to Langmuir adsorption isotherm describes that nature of adsorption of phosphorous in the present study was chemisorption and Freundlich adsorption isotherm is fitted for all the soil since it assumes that the adsorbent has a heterogeneous surface with a varying affinity for P adsorption. Langmuir -2 was identified as the most suitable model for describing P adsorption, followed by Freundlich and Temkin. The OC had a significant positive correlation with maximum quantity adsorbed, KF and qm, while pH had a significant negative correlation with buffer power, maximum quantity adsorbed, KF and qm. Clay had a positive, and sand had a negative correlation with buffer power, maximum quantity adsorbed, KF, KT and qm. Available P was negatively correlated with maximum quantity adsorbed, KF, KT, KL, qm and positively correlated with 1/n. Kuttanad soil recorded the lowest desorption range (0.90-4.54 per cent). The Northern coastal plain reported the lowest dm (8.94 mg kg-1). The desorption ratio ranged from 0.95 -15.30 per cent. The mean value of desorption had a negative correlation with OC, silt, clay, PFC, Ex. Al, maximum quantity adsorbed and qm. The mean value of desorption had a positive correlation with DPS and Av. P. Desorption ratio (Dr) negatively correlated with OC, Ex. Al, maximum quantity adsorbed and qm while Dr had a positive correlation with 1/n. An incubation study was conducted from September to December 2023 to assess the change in pH, EC, Av. P and different fractions of P over 75 days, in which the main and interaction effect of soil types, levels of P and incubation periods were considered. Soil types had a significant effect on pH, EC, Av. P and different fractions of P. Levels of P significantly influenced pH, EC, Av. P and all fractions of P except reductant soluble phosphate while days of incubation had no significant influence on pH. The main effect and interaction effect of soil type, levels of P and days of incubation were found significant for EC, and all P fractions except reductant soluble P. High precipitation of iron and aluminium phosphate was observed on the 15th day of incubation compared to the 45th and 75th days. Meanwhile, the reductant soluble phosphate recorded the highest value on the 75th day of incubation. Elovich kinetic model, Parabolic diffusion model, and Power function model were used to understand the kinetics of phosphorus adsorption in soils with high and low P fixing capacity. 92.34 and 33.12 per cent of applied P were observed to be adsorbed on Kole and Onattukara sandy soil in 120 hours, indicating fast adsorption of high P fixing soil. The Elovich kinetic model best explained the kinetics of high P fixing soil, while the kinetics of low P fixing soil of Onattukara sandy plain best fit the Parabolic diffusion model. The kinetic models could elucidate that the surfaces of the high P-fixing soil exhibited heterogeneity, featured chemisorption and displayed a notably high initial rate of adsorption. The main rate-limiting step of high P fixing soil involved surface adsorption and intraparticle diffusion. Conversely, in the case of low phosphorus fixing soil, it was solely intraparticle diffusion that served as the primary rate-limiting step. Phosphorus retention and availability in different soil types is mainly influenced by pH, AEC, amount of soil separates, organic matter, and amorphous and crystalline forms of Fe and Al oxides. The hysteresis effect was noted high in the soils due to the chemical nature of the adsorption reaction. Phosphorus occluded in amorphous forms of Fe and Al contributes to the plant's available pool. Occlusion of P to Fe and Al oxides was a slow process compared to the precipitation of Fe and Al phosphates in these soils. The chemistry of interaction between organic matter and phosphorus has to be well studied at the structural level. Specific management strategies for phosphorus have to be evolved for similar soil types as the behaviour of P varies widely according to the soil type.